This invention relates generally to the field of well drilling and, in particular, to installation of casing or liners into oil and gas wellbores. Specifically, the invention is an improved method of flotation of these well tubulars into highly deviated wellbores.
Tubular conduits, such as casing, liners or sand exclusion devices, often need to be inserted into a portion of the borehole during drilling or well, completion. In some cases, insertion of these tubular conduits is problematic because of the significant drag forces created by contact between the conduit and the walls of the borehole. Borehole characteristics that tend to result in such detrimental contact are high deviation (measured from the vertical/gravity axis), extended horizontal reach (relative to the surface location of the well or mudline location of the well in the case of an offshore well), and a subsurface trajectory that features frequent or relatively severe changes in well angle or direction.
Numerous problems result from excessive contact between the conduit and the walls of the borehole. This contact creates frictional drag, which increases the downward force necessary to install the conduit. If sufficient additional axial force cannot be applied, the result will be a stuck conduit and possible effective loss of the well. The application of additional axial force can also result in damage to the conduit itself (deformation, buckling, and possibly rupture).
Another problem associated with excessive contact between the conduit and the borehole walls is that the conduit may become xe2x80x98differentially stuckxe2x80x99. This occurs when the conduit makes contact with the wall of the borehole in a permeable section of the formation. The pressure differential between the fluids in the borehole and the fluids in the formation results in a pressure force, which acts to push the conduit toward the borehole wall with which it is in contact. This pressure differential increases the downward force required to push the conduit further into the borehole, with the same resulting problems as those associated with significant frictional drag.
Common installation methods include attempts to overcome or minimize the problems caused by significant conduit to borehole wall contact through the use of low-density fluids to create buoyancy in the deeper section of the conduit. These known string flotation methods require added delay and well completion steps in order to avoid having a loss of well pressure or xe2x80x98kickxe2x80x99 when removing the low-density fluids from the conduit. Such prior attempts are disclosed in U.S. Pat. No. 3,526,280 (Aulick), U.S. Pat. No. 4,384,616 (Dellinger), and U.S. Pat. No. 5,117,915 (Mueller).
As is illustrated in U.S. Pat. No. 3,526,280 (Aulick) a related well completion operation is outlined therein for highly deviated wells. Cement slurry is first pumped down into the borehole to partially displace and replace the mud slurry. The lower portion of the casing string, with a float shoe (and optionally a float collar) at the bottom end, is filled up with fluid (liquid or gas, including air) of lower density than the cement slurry, thereby providing a buoyancy effect to the lower chamber of the casing string. Where it is desirable to confine the buoyant fluid within only a portion of the casing string, a retrievable bridge plug may be positioned a substantial distance above the float shoe. Centralizers are further provided throughout the length of the casing string to minimize contact of the casing string to the borehole wall. Once the casing string has been inserted to the desired depth, the equalizing valve in the bridge plug is opened to allow the fluid above the bridge plug into the buoyancy section. The low-density fluid flows out of the buoyancy section, through the equalizing valve and up the casing string.
A similar well completion operation is illustrated in U.S. Pat. No. 5,117,915 (Mueller). This process attaches a float shoe/float collar to the end of a section of casing string. A buoyant xe2x80x9cfloatingxe2x80x9d portion of the casing string is created by trapping air between the float shoe/float collar and a shear-pinned plug insert. This insert includes a releasable plug (attached by a first set of shear pins) to block a passageway in the body of the insert and contain the air in the buoyancy-aided section of the casing string. Once the casing string has been inserted to the desired depth, the releasable plug in the shear-pinned plug insert is opened to allow the fluid above the plug insert to flow into the buoyancy section. The low-density fluid (air) flows out of the buoyancy-aided section, through the equalizing valve and up the casing string. While Mueller makes no suggestion of the use of centralizers and limits the low-density fluid to air, the thrust of the method is the same as in Aulick and shares the same deficiencies.
The two major deficiencies in both the Aulick and Mueller methods involve the removal of the low-density fluids used to create buoyancy. Significant delays can be created by waiting for the low-density fluid to rise to the top of the casing string. In addition, if the buoyed section is highly deviated, as in the case of a horizontal production well, the light fluid may not migrate up the tubular for removal, as noted by Mueller. Incomplete removal of the low-density fluid results in problematic loss of borehole pressure, described more fully below, as the fluids are eventually released into the annulus between the conduit and the borehole walls.
The method illustrated in U.S. Pat. No. 4,384,616 (Dellinger) also teaches the use of buoyancy-aided insertion of well casing. After providing a means to plug the ends of a pipe string portion, the plugged portion is filled with a low-density, miscible fluid. Once the pipe string has been inserted to the desired depth, the plugs are drilled out and the low-density miscible fluid is forced into the annulus between the pipe string and the wellbore. The low-density fluid must be miscible with the wellbore fluids and the formation to avoid a burp or xe2x80x9ckickxe2x80x9d to or from the formation outside the pipe string. If the light fluid is not miscible with respect to the mud in the borehole and is circulated down the tubular conduit through the lower plug into the casing-by-borehole annulus for the purpose of removal, the lower density of the light fluid will reduce the pressure in the borehole relative to the borehole formation pressure. This can lead to a problematic influx of formation fluid into the borehole. If the light fluid is a gas, and this light fluid is similarly circulated into the casing-by-borehole annulus, the gas can also transmit pressure along the length of the gas bubble, which can be further problematic from a well control perspective, and must be circulated out, requiring no further progress in borehole construction until the gas is circulated up the conduit-by-borehole annulus to the surface. For wells of great depth the time required to make this circulation can be significant. The added expense and difficulties of filling the entire buoyant section with low-density miscible fluid have apparently resulted in little or no commercially practical application of this buoyancy-aided insertion method.
Another buoyancy-aided method used to install tubulars in boreholes that feature these characteristics is to fill an annulus between a concentric insertion tubular string and the casing (or liner) with a fluid (a liquid or a gas) that has a lower density than the liquid contained inside the borehole. Similar to the methods described above, buoyancy created by the difference in the fluid density in the insertion-string-by-casing annulus and the density of the fluid in the borehole reduces the net weight of the tubular section as it is inserted into the borehole. The main advantage gained by use of the annulus buoyancy chamber method is that it allows drilling mud to be circulated, through the insertion string, during insertion or other operations. This method is also described in detail in U.S. Pat. No. 5,117,915 (Mueller).
Accordingly, there is a need for a tubular insertion methodology that will enable buoyancy-aided insertion of tubulars within a wellbore while avoiding the added expense, complexities and delays inherent in the currently known methods.
This invention provides a method for buoyancy-aided insertion of a tubular conduit into a borehole by removing the fluids from a section of the conduit, thus creating at least a partial vacuum in a section of the conduit. The density difference between the fluid residing in the borehole and the evacuated conduit section results in partial or full buoyancy of the evacuated section of tubular conduit. A preferred embodiment is to form this vacuum between a lower plug and an upper plug in the conduit, or in the annulus between an insertion string and the conduit, between lower and upper annular plugs. The terms xe2x80x98upperxe2x80x99 and xe2x80x98lowerxe2x80x99 refer to the plugs"" relative location while the conduit is within the vertical section of the borehole, the plugs keep their respective labels even under borehole deviation greater than 90 degrees. Once the tubular is in place, the barrier between the evacuated section and the borehole or insertion string fluids is eliminated, allowing these fluids to fill the evacuated interval. These fluids would then be replaced from the surface, with no need to remove any low-density fluid through the conduit or the borehole.